A known four-wheel-drive vehicle is equipped with a driving force switching device by which a two-wheel-drive state is selectable in response to a road status or a running status of a vehicle for the purposes of improving a fuel consumption, or a four-wheel-drive differential lock function is turned on for the purposes of improving a running through performance when a vehicle is running in the mud.
According to such a driving force switching device, a sleeve having a spline gear or a dog clutch arranged on a drivetrain performs a stroke operation by means of a motor-driven actuator so as to engage or disengage a driving shaft and a driven shaft with each other for the purposes of transmitting or interrupting a driving force.
A two-wheel-drive/four-wheel-drive switching device is disclosed in JP3521945B2. The two-wheel-drive/four-wheel-drive switching device is employed in an actuator that includes a spiral spring disposed between a first plate rotatable as a unit with a motor-driven gear and a second plate rotatable as a unit with an output shaft. The actuator further includes a bush member including a groove engaging with a bending inner end portion of the spiral spring and a diameter extending portion in contact with the first plate and the second plate on an outer circumferential side of the spiral spring. When the first plate rotates in one direction (i.e. clockwise direction) by means of a driving of the motor, an outer end portion of the spiral spring is pressed in the clockwise direction so that the spiral spring is wound. The rotation of the first plate then causes the second plate to rotate by means of the inner end portion of the spiral spring, the groove and the diameter extending portion of the bush member. At this time, if a shift fork for moving a sleeve is not capable of performing a stroke operation since phases of respective spline gears of a driving shaft and a driven shaft of a transfer device are not matched with each other and then the output shaft of the actuator cannot rotate, the rotation of the first plate is stored in the spiral spring as deflection energy. Further, when the first plate rotates in the other direction (i.e. counterclockwise direction) by means of a driving of the motor, the diameter extending portion of the bush member is pressed in the counterclockwise direction so that the spiral spring is wound from the inner end portion by means of the groove of the bush member engaging with the inner end portion of the spiral spring. The second plate is rotated by the outer end portion of the spiral spring accordingly. At this time, if the output shaft cannot rotate in the same way as the first plate rotating in one direction, the rotation of the first plate is stored in the spiral spring as deflection energy. That is, the actuate is equipped with a standby mechanism by which a relative rotation between the first plate and the second plate can be stored in the spiral spring as deflection until the output shaft is brought to be able to rotate. The deflection stored is released when the output shaft is able to rotate, thereby causing the sleeve to perform a stroke operation by means of the shift fork connected to the output shaft. The driving shaft and the driven shaft are connected to each other so that a switching from the two-wheel-drive state to the four-wheel-drive state or vice versa can be performed.
Further, another actuator is disclosed in JP2003-336717A. The actuator disclosed includes an input side rotating member, an output side rotating member, a spring, a first spring receiving member, and a second spring receiving member all rotatably arranged on an identical rotational center. The actuator further includes an input rotation directly transmitting portion disposed between the first spring receiving member and the second spring receiving member for directly transmitting, not via the spring, a rotational force of a driving shaft to a driven shaft by the first spring receiving member and the second spring receiving member attaching to each other when the first spring receiving member and the second spring receiving member relatively rotate to each other by a predetermined angle or more.
In order to achieve a reduction in size and weight of the actuator for the purposes of improving a mounting ability of the actuator in a vehicle, or to realize a high-power of the actuator without changing a size thereof, however, the following issues may occur.
According to the actuator disclosed in JP3521945B2, the spiral spring is disposed between the first plate and the second plate. Since a rotating torque of the first plate is always transmitted to the second plate by means of the spiral spring, a maximum output load of the actuator is equal to spring characteristics (deflection-load characteristics) of the spiral spring. Thus, the maximum output is determined on the basis of a load of the spiral spring. That is, in case of attempting a reduction in size and weight of the actuator without changing an output load of the actuator, a restriction exits on a downsizing of the entire actuator since a downsizing of the spiral spring is limited so as to maintain the output load. On the other hand, improving the output load of the actuator without changing a size thereof means increase in size of the spiral spring.
The actuator disclosed in JP2003-336717A includes as the standby mechanism the first spring receiving member, the second spring receiving member, and the input rotation directly transmitting portion for directly transmitting, not via the spring, a rotational force of the driving shaft to the driven shaft by attaching the first spring receiving member and the second spring receiving member to each other when the first spring receiving member and the second spring receiving member relatively rotate to each other by a predetermined angle or more. Then, the output load of the actuator is improved by using the output load of the motor so that a power switching operation can be surely performed.
According to the aforementioned standby mechanism, however, the input side rotation member and the spring receiving member are formed separately, and then the input rotation directly transmitting portion is formed as a fan-shaped projection in an axial direction of the spring receiving member. The standby mechanism has a certain degree of thickness in the axial direction and consists of a combination of great number of parts. That is, the actuator includes a great number of components and is large in size in the rotational axis direction.
Thus, a need exists for an actuator which can provide an increased maximum output load and improve certainty of switching of driving force while achieving an entire reduction in size and weight with avoiding a complexity of structure.